Miniature loudspeakers are widely used in a variety of small portable devices, such as mobile phones, music players, personal digital assistants, hearing aids, earphones, portable ultrasonic equipment, and so forth, where small dimensions are paramount. Users of such devices appreciate their small dimensions, but would prefer not to compromise regarding sound quality. However, these devices are typically battery operated, which further limits the amount of electrical power available to drive the miniature loudspeaker. Also the fact that many of these applications are very sensitive to price dictates that production costs should be very low. Very often the life cycle of such products is very short, thus the design time of new products should be very short.
Today many of these miniature loudspeakers are driven by analog class A/B amplifiers connected to the loudspeaker with external connections. These analog class A/B amplifiers are bulky, inefficient, costly etc. Even further, there are many constraints if one wants to standardise their usage, i.e. interface etc.
Electro-Magnetic Interference (EMI) is becoming an even more increasing problem within microelectronics, thus causing problems with poor noise performance. This calls for solutions suited for integration of the loudspeaker driver into the miniature loudspeaker. By integrating the active signal processing circuit into the miniature loudspeaker casing, the circuit can effectively be shielded against EMI. Thus, there is a need for a digital driver which can be implemented with minimum physical size without decreasing the performance of the driver. Furthermore, such drivers must be suited for low cost production.
The most natural solution is to replace the analog amplifiers with digital driver circuits which can be made highly efficient, fairly small, and with very high quality. Furthermore, when using digital driver circuits, standard digital interface is very easily implemented.
Several solutions on the issue of replacing power amplifiers with digital driver circuits already exist in numerous prior art documents. Examples of such documents are: U.S. Pat. No. 5,077,539 from Apogee Technology, and U.S. Pat. No. 5,777,512 from Tripath Technology, and U.S. 2002/0075068 A1 from Wei-Chan HSU.
The above-mentioned documents aim at applications with power levels of several Watts, such as Hi-Fi sound quality systems. Furthermore, these solutions are quite complex, and they often require many external components thus being too costly to implement in high volume low cost applications. Furthermore, if the driver circuit is to be integrated into the miniature loudspeaker then it is of paramount importance that the physical size of the circuit including external components is as small as possible. None of the above mentioned solutions fulfils this criteria.
U.S. Pat. No. 5,815,581 from Mitel Semiconductor and U.S. Pat. No. 6,191,650 from G/N Netcom describe drivers for hearing aids comprising class D amplifiers in combination with Pulse Width Modulation (PWM). Both of these solutions feature feedback loops for minimizing distortion. Since the inventions described in U.S. Pat. No. 5,815,581 and U.S. Pat. No. 6,191,650 are intended for use within hearing aids, they are suited for miniature applications. However, the circuit structures are rather complex, and thus not suited for low cost production.
Several of the above-mentioned prior art documents describe three-level sigma-delta modulation based drivers or amplifiers, which offer superior efficiency compared to two level (1-bit) sigma-delta modulation systems. Normally, a three-level driver is combined with PWM.
DE 44 41 996 A1 describes a two-level Pulse Density Modulation (PDM) driver for hearing aids. PWM is more complicated to implement but can be operated at a lower clock frequency than PDM, which is an advantage as the H-bridge converts the digital signal into an analog output signal with less error if the clock frequency is lower. The lower complexity of the PDM is very attractive for high volume applications, as the lower complexity will result in lower production cost. However, it is generally known that PDM implementations require a very high clock frequency which has disadvantages such as distortion due to switching rise and fall times, and if standard components are used switching loss will result in decreased efficiency.
Thus, there is a need for a miniature loudspeaker driver offering high efficiency, small dimensions, is suited for low cost production, and still with high quality performance regarding Signal-to-Noise-Ratio (SNR) and distortion.
It is an object of the present invention to provide a driver for digitally converting a signal into a modulated signal with the lowest possible clock frequency, the lowest complexity, and still achieving good performance.
It is a further object of the present invention to combine the driver with a miniature loudspeaker in a complete system thus achieving the smallest possible size, thereby making the system suitable for applications with very limited space available.
It is a still further object of the present invention to provide a miniature loudspeaker assembly with minimal emission of EMI due to the integrated, shielded and dense nature of a miniature assembly also leading to a low susceptibility to EMI.